747-400F also has a weight & balance computer. Tolerance is about 2%; given that mtow is almost 400T, you could be 8T out. Discrepancies of 4-5T are investigated prior dispatch. One other detail is that for the system to work properly, the aircraft has to be on a flat bit of tarmac; slopes affect the readings.
At the end of the day, proper care and attention must be given when issuing a loadsheet. There really is little else to say.